Damn, I've been playing this for too long now.
Canabalt is a very simple game. You press X or C to start the game. Your protagonist starts running in a corridor in some office skyscraper. Then he bursts out through the window at the end and starts running on rooftops (and occasionally through skyscrapers). Press X or C to jump, or the little guy will fall to his death.
Meanwhile, giant robots patrol in the background, fighter planes zoom past right behind you and occasionally, a missile crashes ahead of you - necessitating a jump to avoid being vaporized. There is no explicit story, but the implied one is rather obvious - WWIII has started, and you've decided to run out of the city as fast as you can.
One button. And I've been playing this for over a week now. I can consistently get over 2000 meters as long as a window doesn't brutally stop me before that.
Canabalt is awesome and will eat all your free time. You have been warned.